The Zudong heavy rare earth element (HREE) deposit in South China is the largest regolith-hosted HREE deposit in the world, with a resource of ~17,600 tonnes (t) of rare earth oxides (REOs) at an average grade of ~0.1 wt % REOs. Despite more than 40 years of exploration and exploitation, the genesis of this deposit is poorly understood. Subtropical weathering of the parent A-type granite formed orebodies hosted mainly within the lower B to upper C horizons of the resulting soil, which is developed on the hillsides of a moderately incised landscape (relief ~150 m). The thickness of the orebodies varies from a few meters to up to 10 m. REE concentrations increase from ~300 ppm in the A horizon to ~1,500 ppm in the lower B to upper C horizons and decrease with further depth in the profile. The entire soil profile is enriched in HREEs, with (La/Yb)N ratios <1, but the REE-rich lower B horizon is less enriched in HREEs, with (La/Yb)N values up to 0.9, than the underlying upper C horizon. Exchangeable REEs, representing REEs that are adsorbed in the deposit, constitute ~65% of the bulk REE content; the light REEs (LREEs) are preferentially adsorbed. The main minerals adsorbing the REEs are kaolinite and halloysite; the proportions of REEs adsorbed by Fe-Mn oxyhydroxides and organic matter are negligible. The remaining REE content is hosted by supergene chernovite (YAsO4) and residual aeschynite/euxenite-group and fergusonite-group minerals, xenotime-(Y), zircon, and thorite.

The Zudong deposit formed from weathering of a HREE-rich A-type granite, containing between ~200 and ~450 ppm REEs dominantly as synchysite-(Y), yttrian fluorite, gadolinite-(Y), hingganite-(Y), and yttrialite-(Y), which are susceptible to chemical weathering. Decomposition of these minerals released REEs to the weathering fluid, and the interaction between alkali feldspar, muscovite, and the weathering fluid produced halloysite and subordinate illite and smectite. With progressive weathering, these minerals accumulated together with quartz, refractory zircon, xenotime-(Y), and aeschynite/euxenite-group and fergusonite-group minerals. In the upper parts of the soil profile, where conditions were acidic, dissolution of REE minerals is promoted, and the REEs tend to remain in solution. Deeper in the profile (lower part of the B horizon and upper part of the C horizon), conditions become less acidic, leading to adsorption of the REEs and As (released by weathering from an adjacent coal unit) on the surface of halloysite. With time, halloysite, a metastable phase in respect to kaolinite, gradually transforms to the latter, releasing As and variable proportions of the adsorbed REEs. The released REEs and As are transported downward, partially readsorbed by clay minerals, and the remaining REEs combined with As to precipitate supergene chernovite-(Y). Ongoing operation of this eluviation-illuviation process at Zudong has gradually enriched the soils in HREEs to form a resource that now supplies much of the world’s HREEs.

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